In recent years, surface-treated steel sheets produced by rustproofing steel sheet materials, particularly rustproof hot-dip galvanized steel sheets and galvannealed steel sheets, have been used in the fields of automobiles, household electrical appliances, and construction materials.
In general, hot-dip galvanized steel sheets are produced by the following method. First, a slab is subjected to hot rolling, cold rolling, or heat treatment to form a thin steel sheet. The surface of the steel sheet is degreased and/or pickled in a pretreatment step. Alternatively, without the pretreatment step, oils on the surface of the steel sheet are burned in a preheating furnace. The steel sheet is then heated in a nonoxidizing or reducing atmosphere for recrystallization annealing. The steel sheet is then cooled in a nonoxidizing or reducing atmosphere to a temperature suitable for coating and is immersed in a hot-dip galvanizing bath without exposed to the air. The hot-dip galvanizing bath contains a minute amount of Al (approximately 0.1% to 0.2% by mass). Thus, the steel sheet is coated and becomes a hot-dip galvanized steel sheet. Galvannealed steel sheets are produced by heat-treating hot-dip galvanized steel sheets in an alloying furnace.
In recent years, in the automotive field, steel sheet materials have had higher performance and reduced weight. Strength reduction resulting from weight reduction of steel sheet materials is compensated for by the addition of solid-solution strengthening elements, such as Si and Mn. In particular, Si can advantageously reinforce steel without decreasing ductility. Thus, Si-containing steel sheets are promising high-strength steel sheets. However, the following problems occur in the production of hot-dip galvanized steel sheets and galvannealed steel sheets based on high-strength steel sheets containing large amounts of Si.
As described above, hot-dip galvanized steel sheets are annealed in a reducing atmosphere before coating. However, because of its high affinity for oxygen, Si in steel is selectively oxidized even in a reducing atmosphere and forms oxides on the surface of steel sheets. These oxides decrease the wettability of the surface of the steel sheets and form uncoated areas in a coating operation. Even when uncoated areas are not formed, these oxides decrease the adhesiveness of the coating.
Furthermore, these oxides significantly decrease the alloying speed in an alloying process after hot-dip galvanizing. This greatly decreases the production of galvannealed steel sheets. Alloying treatment at high temperatures for the purpose of high productivity may lower powdering resistance. Thus, it is difficult to achieve efficient production and high powdering resistance at the same time. Alloying treatment at high temperatures makes the retained austenite phase unstable and reduces the advantage of the addition of Si. Thus, it is very difficult to produce high-strength hot-dip galvanized steel sheets that have good mechanical characteristics and coating quality at the same time.
Several techniques are disclosed in order to address these problems. Patent Literature 1 discloses a technique for improving the wettability of a steel sheet to molten zinc by forming iron oxide on the surface of the steel sheet in an oxidizing atmosphere and then forming a reduced iron layer on the surface of the steel sheet by reduction annealing. Patent Literature 2 discloses a technique for ensuring high coating quality by controlling the atmosphere, such as the oxygen concentration, in a preheating operation. Patent Literature 3 discloses a technique of producing a hot-dip galvanized steel sheet that has no uncoated area and has good appearance by dividing the heating zone into three zones A to C and appropriately controlling the temperature and oxygen concentration of each of the heating zones to reduce the occurrence of indentation flaws.